SIRT3 gene expression but not SIRT3 subcellular localization is altered in response to fasting and exercise in human skeletal muscle

New Findings: What is the central question of this study? Evidence from cellular and animal models suggests that SIRT3 is involved in regulating aerobic ATP production. Thus, we investigated whether changes in fatty acid and oxidative metabolism known to accompany fasting and exercise occur in association with changes in SIRT3 mitochondrial localization and expression in human skeletal muscle. What is the main finding and its importance? We find that 48 h of fasting and acute endurance exercise decrease SIRT3 mRNA expression but do not alter SIRT3 mitochondrial localization despite marked increases in fatty acid oxidation. This suggests that SIRT3 activity is not regulated by changes in mitochondrial localization in response to cellular energy stress in human skeletal muscle. The present study examined SIRT3 expression and SIRT3 mitochondrial localization in response to acute exercise and short-term fasting in human skeletal muscle. Experiment 1 involved eight healthy men (age, 21.4 ± 2.8 years; peak O₂ uptake, 47.1 ± 11.8 ml min⁻¹ kg⁻¹) who performed a single bout of exercise at ∼55% of peak aerobic work rate for 1 h. Muscle biopsies were obtained at rest (Rest), immediately after exercise (EX-0) and 3 h postexercise (EX-3). Experiment 2 involved 10 healthy men (age, 22.0 ± 1.5 years; peak O₂ uptake, 47.2 ± 6.7 ml min⁻¹ kg⁻¹) who underwent a 48 h fast, with muscle biopsies collected 1 h postprandial (Fed) and after 48 h of fasting (Fast). Mitochondrial respiration was measured using high-resolution respirometry in permeabilized muscle fibre bundles to assess substrate oxidation. Whole body fat oxidation increased after both exercise (Rest, 0.96 ± 0.32 kcal min⁻¹; Exercise, 5.66 ± 1.97 kcal min⁻¹; P < 0.001) and fasting (Fed, 0.87 ± 0.51 kcal min⁻¹; Fast, 1.30 ± 0.37 kcal min⁻¹, P < 0.05). SIRT3 gene expression decreased (P < 0.05) after both exercise (−8%) and fasting (−19%); however, SIRT3 whole muscle protein content was unaltered after fasting. No changes were observed in SIRT3 mitochondrial localization following either exercise or fasting. Fasting also decreased the V_max of glutamate [80 ± 43 versus 50 ± 21 pmol s⁻¹ (mg dry weight)⁻¹; P < 0.05]. These findings suggest that SIRT3 does not appear to be regulated by changes in mitochondrial localization at the time points measured in the present study in response to cellular energy stress in human skeletal muscle.